1. Field of the Invention
The present invention relates to the detoxification of aqueous waste streams and, more particularly, to the degradation of reducible organic compounds in aqueous waste streams to provide effluents containing innocuous degradation products.
2. Description of the Prior Art
Many halogenated organic compounds such as those used for pesticides, herbicides, dielectric oils and flame retardants are very stable and consequently they accumulate in the environment. Some are toxic even at low levels while others are accumulated in organisms with concentrations increasing with the level in the food chain until higher animals may be rendered unfit for human consumption or may be toxic to higher species. Careful controls during the manufacture and utilization of such halogenated organic compounds have reduced their entry into the environment. However, these effluent streams still generally contain 1 ppm or less of halogenated toxicant which is at a level harmful to the environment. Furthermore, these streams are difficult to treat because of the low concentration of these extremely stable compounds. No cost effective means has been developed for removal of these materials from solution in aqueous waste streams necessarily generated during manufacture and use. Although the solubilities of such compounds in water are generally less than 1 ppm, this level is considered excessive for discharge.
It has recently been found that in addition to compounds of the more complex type mentioned above, low molecular weight halogenated hydrocarbons ("light ends" e.g. chloroform and similar halogenated compounds) appear in trace amounts in river systems. Certain of these compounds, presumably generated by the chlorination of municipal or industrial waste water, are suspected carcinogens.
Other reducible organic species are responsible for severe contamination, and are in some cases carcinogenic. Among the types of compounds recognized as requiring removal from waters are the nitros mines, phenols, nitrogen compounds and heterocyclics such as the s-triazines.
The reductive degradation of chlorinated hydrocarbons by means of metals or metallic couples has been disclosed in U.S. Pat. Nos. 3,640,821; 3,737,384; 3,767,782; 3,767,783; 3,871,906; and 3,873,346. Degradation of the chlorinated hydrocarbons and their consequent detoxification occurs as a result of removal of chlorine from the organic systems with the formation of metal chlorides. The latter patents relate to self-destructing pesticides in which the pesticide is adhered to a coated metal particle. The coating produces the acid medium (pH 1.5 to 4) preferred in all these patents. The earlier patents suggest the treatment of acidic aqueous streams with a slight stoichiometric excess of metal for effective reaction such as at least about 2:l (weight basis about 0.5:l) the highest ratio being about 3.6:l (1:1 weight basis).
The use of acidic conditions required in these prior patents to treat an aqueous waste stream is accompanied with an excess dissolution of the metal catalyst reductant which is costly and results in an excessive quantity of metal ions in the effluent stream, which, in itself would be unsatisfactory for discharge to the environment, even with iron, the least toxic metal employed.